Tuesday, February 28, 2012

Flowering plants in the sea part 2 - Sex

A while ago I wrote about seagrass and some of the interesting adaptations that they have to the low light levels in the sea. Another important part of their biology that had to adapt to conditions in the marine environment were their flowers. Because seagrasses moved into the sea on multiple occasions there are several strategies that they use for pollination.

The most recent entrants to the sea, in the genus Enhalus, have flowers that are pollinated in air. Male flowers break away from the plant and fertilise the female flowers, which are attached to the plant by long coiled tendrils. Unlike many other types of seagrass, the female flowers are easily recognised as flowers.

The floating female flower of Enhalus acroides. The small white polystyrene bead-like objects are the male flowers (photo The Tide Chaser). 
A close up of the male flowers of E. acroides (photo Urban Forrest).

Pollination is important in terrestrial plant populations, but was thought not to be terribly important in seagrass populations because they are mostly clonal and populations expand by vegetative growth. Indeed, expansion via the rhizomes can produce large seagrass meadows that contain genetically very similar plants.

The female flower of surfgrass, Phyllospadix torreyi  (photo Carol Blanchette).
The flower of eelgrass, Zostera marina (photo Jan Holmes)

Seagrasses also invest a lot of energy into sexual reproduction. This is curious because it takes energy away from vegetative growth, which is primarily how meadows are maintained and recover from damage. Indeed, sex seems paradoxical in species, like segrasses, that can produce offspring without sex.

One potential reason for the large investment in sexual reproduction is seagrasses is long range dispersal. Vegetative growth expands meadows, and can do so quite rapidly, but it can't jump large gaps and establish new populations or spread genes to new locations. Dispersing pollen and seeds may be able to accomplish this.

The pollen of seagrasses is large and elongate (relative to other flowering plants), and consequently, poorly suited to long distance travel. Although estimates are rare, some studies suggest that pollen may only be able to travel a few tens of meters. Pollination, therefore, is likely to occur at local scales in most seagrasses.

The fruit of eelgrass, Zostera marina. Each fruit contains a single seed (photo Jan Holmes).
The seeds of seagrasses have the potential to disperse genes much further than the pollen and establish new meadows. Seagrass seeds, or the structures that carry the seeds (e.g. fruit) have a variety of adaptations for dispersal that effect how far the seeds will travel before they start a new population. Probably the most important factor determining dispersal distance.

The seeds themselves are usually neutrally or negatively buoyant because they must eventually reach the sediment to grow into an adult plant. The structures that carry the seeds, however, are often floating and can transport the seeds considerable distances (up to several hundred kilometers). Some seagrasses are even able to transport their seeds in the insides of herbivores like dugongs and turtles.

Dugongs are mostly interesting because they transport seagrass seeds.
Another reason that sex is important for seagrasses is the resilience of populations to disturbance. We know that communities with a greater diversity of species often have an enhanced ability to resist and recover from disturbances. Interestingly, seagrass patches that have higher genetic diversity show a greater resistance to damage by herbivores and recover faster after damage. Although, the faster recovery may be due to the lower levels of damage in more diverse patches than faster rates of vegetative growth.

Further reading:

1 G. A. Kendrick et al. (2012). The Central Role of Dispersal in the Maintenance and Persistence of Seagrass Populations BioScience, vol 62(1): 56-65

2 J. D. Ackerman (2006). Sexual Reproduction of Seagrasses: Pollination in the Marine Context. In: Seagrasses: Biology, Ecology and Conservation (A. W. D. Larkum, R. J. Orth, C. Duarte Eds.). 89 - 109

3 R. J. Orth et al. (2006) Ecology of Seagrass Seeds and Dispersal Strategies. In: Seagrasses: Biology, Ecology and Conservation (A. W. D. Larkum, R. J. Orth, C. Duarte Eds.). 111 - 133

4 A. R. Hughes and J. J. Stachowicz (2004)Genetic diversity enhances the resistance of a seagrass ecosystem to disturbance. PNAS vol 101(24): 8998 - 9002.

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